Bottom Line:
When the muscle-tendon unit received eccentric cyclic loading to 112%, changes in all biomechanical parameters - except for the slope of the load-deformation curve - were not significant.In contrast, most parameters, including the slope of the load-deformation curve, peak load, deformation at peak load, total energy absorption, and energy absorption before peak load, significantly decreased after isokinetic eccentric cyclic loading to 120%.We found a threshold for eccentrically induced injury of the rabbit triceps surae muscle at between 12% and 20% strain, which is within the physiologic excursion of the muscle-tendon units.

Background and purpose: Intensive eccentric exercise can cause muscle damage. We simulated an animal model of isokinetic eccentric exercise by repetitively stretching stimulated triceps surae muscle-tendon units to determine if such exercise affects the mechanical properties of the unit within its physiologic excursion.

Methods: Biomechanical parameters of the muscle-tendon unit were monitored during isokinetic eccentric loading in 12 rabbits. In each animal, one limb (control group) was stretched until failure. The other limb (study group) was first subjected to isokinetic and eccentric cyclic loading at the rate of 10.0 cm/min to 112% (group I) or 120% (group II) of its initial length for 1 hour and then stretched to failure. Load-deformation curves and biomechanical parameters were compared between the study and control groups.

Results: When the muscle-tendon unit received eccentric cyclic loading to 112%, changes in all biomechanical parameters - except for the slope of the load-deformation curve - were not significant. In contrast, most parameters, including the slope of the load-deformation curve, peak load, deformation at peak load, total energy absorption, and energy absorption before peak load, significantly decreased after isokinetic eccentric cyclic loading to 120%.

Conclusion: We found a threshold for eccentrically induced injury of the rabbit triceps surae muscle at between 12% and 20% strain, which is within the physiologic excursion of the muscle-tendon units. Our study provided evidence that eccentric exercise may induce changes in the biomechanical properties of skeletal muscles, even within the physiologic range of the excursion of the muscle-tendon unit.

Figure 1: Representative load-deformation curve of a triceps surae muscle-tendon unit after isokinetic eccentric cyclic loading for 1 hour at 12% strain. The curve shows a slope of 54.9 N/mm for the study group, compared with 36.5 N/mm for the control sample.

Mentions:
After isokinetic eccentric loading, all muscle-tendon units under stretch had similar curve patterns. The load-deformation curve began with an initially increasing slope and ultimately reached the peak load. After this point, a steep decline was observed, followed by a curve with gradual increasing and decreasing of the load. After 12% strain for 1 hour, the curve shows a slope of 54.9 N/mm for the study group, compared with 36.5 N/mm for the control sample. The slope of the curve was steeper in the study group than in the control group (Fig. 1). When the muscle-tendon unit was loaded to 20% strain for 1 hour, we observed a significant change on the load-deformation curve between the control and study groups. All biomechanical parameters were substantially decreased in the study group. For the control and study groups, respectively, peak load was 850.5 and 305.4 N, deformation at peak load was 35.93 and 20.9 mm, the slope of the curve was 31.1 and 20.5 N/mm, total energy absorption was 23764.6 and 3989.5 N-mm, and energy absorption before peak load was 11564.5 and 2194.0 N-mm. The peak load was lower in the study group than in the control group (Fig. 2).

Figure 1: Representative load-deformation curve of a triceps surae muscle-tendon unit after isokinetic eccentric cyclic loading for 1 hour at 12% strain. The curve shows a slope of 54.9 N/mm for the study group, compared with 36.5 N/mm for the control sample.

Mentions:
After isokinetic eccentric loading, all muscle-tendon units under stretch had similar curve patterns. The load-deformation curve began with an initially increasing slope and ultimately reached the peak load. After this point, a steep decline was observed, followed by a curve with gradual increasing and decreasing of the load. After 12% strain for 1 hour, the curve shows a slope of 54.9 N/mm for the study group, compared with 36.5 N/mm for the control sample. The slope of the curve was steeper in the study group than in the control group (Fig. 1). When the muscle-tendon unit was loaded to 20% strain for 1 hour, we observed a significant change on the load-deformation curve between the control and study groups. All biomechanical parameters were substantially decreased in the study group. For the control and study groups, respectively, peak load was 850.5 and 305.4 N, deformation at peak load was 35.93 and 20.9 mm, the slope of the curve was 31.1 and 20.5 N/mm, total energy absorption was 23764.6 and 3989.5 N-mm, and energy absorption before peak load was 11564.5 and 2194.0 N-mm. The peak load was lower in the study group than in the control group (Fig. 2).

Bottom Line:
When the muscle-tendon unit received eccentric cyclic loading to 112%, changes in all biomechanical parameters - except for the slope of the load-deformation curve - were not significant.In contrast, most parameters, including the slope of the load-deformation curve, peak load, deformation at peak load, total energy absorption, and energy absorption before peak load, significantly decreased after isokinetic eccentric cyclic loading to 120%.We found a threshold for eccentrically induced injury of the rabbit triceps surae muscle at between 12% and 20% strain, which is within the physiologic excursion of the muscle-tendon units.

Background and purpose: Intensive eccentric exercise can cause muscle damage. We simulated an animal model of isokinetic eccentric exercise by repetitively stretching stimulated triceps surae muscle-tendon units to determine if such exercise affects the mechanical properties of the unit within its physiologic excursion.

Methods: Biomechanical parameters of the muscle-tendon unit were monitored during isokinetic eccentric loading in 12 rabbits. In each animal, one limb (control group) was stretched until failure. The other limb (study group) was first subjected to isokinetic and eccentric cyclic loading at the rate of 10.0 cm/min to 112% (group I) or 120% (group II) of its initial length for 1 hour and then stretched to failure. Load-deformation curves and biomechanical parameters were compared between the study and control groups.

Results: When the muscle-tendon unit received eccentric cyclic loading to 112%, changes in all biomechanical parameters - except for the slope of the load-deformation curve - were not significant. In contrast, most parameters, including the slope of the load-deformation curve, peak load, deformation at peak load, total energy absorption, and energy absorption before peak load, significantly decreased after isokinetic eccentric cyclic loading to 120%.

Conclusion: We found a threshold for eccentrically induced injury of the rabbit triceps surae muscle at between 12% and 20% strain, which is within the physiologic excursion of the muscle-tendon units. Our study provided evidence that eccentric exercise may induce changes in the biomechanical properties of skeletal muscles, even within the physiologic range of the excursion of the muscle-tendon unit.